Many devices are available for providing indications of temperature of surfaces. The most accurate of those devices are generally contact devices such as thermistors or thermocouples which must be placed in physical contact with the surface, a disadvantage in some applications. A further disadvantage of such temperature sensors is that they generally require an amount of time in the order of seconds to stabilize to the temperature of the surface.
Radiation detectors have been used as a noncontact alternative to such temperature sensors. Such detectors are based on the principle that the thermal radiation emitted from a surface is proportional to the temperature of the surface raised to the fourth power. Typically radiation sensors much as thermopiles respond to changes in radiation in the order of one tenth second. Unfortunately, the radiation emitted is also a function of the emissivity of the surface and of background radiation. Because the emissivity of the surface is generally not accurately known assumptions must be made, and those assumptions lead to inaccuracies in the temperature reading.
A more rapid and accurate temperature reading can be obtained with a radiation sensing device which further includes means for eliminating the effects of emissivity on the output. This has been accomplished by means of a high reflectivity hemispherical cup which is placed against the surface. Radiation from the surface is detected through an aperture in the cup. By reflecting all emissions from a target surface back onto that surface, the cup causes the surface to behave as a black body, the emissivity of which is equal to one, regardless of the actual emissivity of the surface. With such an arrangement, the temperature can be rapidly detected by a thermopile which may come to equilibrium in less than a tenth of a second and assumptions with regard to emissivity are unnecessary.